High-frequency transmission



May 27, 1924.

J.; F. FARRINGTON HIGH FREQUENCY TRANSMISSION Filed Feb. 1, 1922 looov 225%. |0698 Q.nl wngmm.. N\

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Patented May 27, 1924.V

.UNIT-.ED STATES 1,495,410 PMENTv OFFICE.

JOHN F. FARRINGTON, OF NEW YORK, N. Y., ASSIGNOR '.lO WESTERN ELECTRIC COM- PANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

HIGnsranpUnNcY TRANSMISSION.

applicati mea remmen;4 1, 1922., serial No. 533,198.

To all 'whom it may coo/wem: i 1

Be itknown that I, JOHN F. FARRIrIG'roN, a citizen of the United States, ,residingl at New York, in the county of Bronx, State of New York, have invented certain .new and useful Improvements in High-Frequency Transmission, of which the following is a full, clear, concise, and exact description.'j

v waves.

' ity of invention,

An object 'ofl the-invention is to decrease the interference vbetween channels of the same or different carrier wave transmission systems and thereby increase the number of possible channels in a multiplexsystemor increase the number of simultaneous conversations which can 'be carried on through the same energy transferring medium as, for example, the ether of a given region. Since the radio field is the principal field of utilradio signaling systems, are describedherein to illustrate the principles involved. g

Another object of the invention is to provide improved methods of and means for secret communication.

In one well-known type of carrier wavesignaling `system two-way communication between distant stations isI carried on by modulating -a high frequency carrier wave at each of the stations in accordance with the signal to be transmitted to the other station.' The carrier waves at the two-stations are not of the same frequency but diier from each other by a dierence frequency preferably above audibility in the case of telephony but not otherwise limited to any definite-range. The receiver at each stal tion receives the carrier wave from its own transmitter as well as that from the distant transmitter. These two different frequencyI received waves are combined to produce a third carrier wave having a frequency withina range preferably of 8.000 to.,100,000 cycles per second but in any case having its frequency dependent upon the frequencies of the two principal carrier waves. The particular range indicated is preferred because it is high enough to act as a carrier for speech frequencies and not high enough to necessitate expensive apparatus for amplification and selection. The wave of the third frequency produced at each station will be styled the auxiliary carrier wave, to distinguish it from the principal carrier waves which are combined to produce it. The auxiliary carrier wave is modulated in accordance with the signal being sent by the distant transmitter when the operator thereat is sending and is modulated in accordance'with the signal of the local transmitter when the local operator is sending. If both are sending, the auxiliary carrier wave will be modulatedin accordance with both signals and both signals will be heard at each station. Both principal carrier waves must be transmitted when signals are transmit- -ted from either station to the other.

In another" well-known type of system 1 a modulated carrier wave having its unmodulated component suppressed is radiated or otherwise transmitted. This is sometimes styled a carrier suppression system and the method employedis called carrier suppression. In the present system a similar principle -is made use of for partial carrier suppression.

A particular object of the present invention is to provide methods and means for combining substantial advantages of the two systems described, that is,-.for utilizing partial carrier suppression in duplex sys- -`tems wherein the lncoming and outgoing carrier waves combine to produce an auxiliary carrier wave at each station.

A-diiiculty which is apparent in applying the method of carrier suppression to twoway systems of the kind described is that the auxiliary carrier frequency at each station will lack an unmodulated component of suiicient amplitude to reproduce a low frequency signaling wave of the original low frequency wave form. See article by Colpitts and Blackwell, Carrier current telegraphy 'and telephony, Proceedings of the American Institute of Electrical'zngineers, April, 192,1, vol. 40, No. 4, page 309. In case the original wave form is not approximately reproduced, the'reproduced signal will not be of good quality. This is especially important in telephony.

As hereinafter described, lthere are provided means and methods for increasing the relative amplitude of the unmodulated component of the auxiliary carrier frequency with respect to its accompanying modulated component.

Use is made of an additional auxiliary carrier wave which is generated at each station of the two-way system. The energy of this wave is utilized 'at the transmitter in transmission and atI the receiver in reception. By means of this additional auxiliary carrier wave, the frequencies of the side ban-ds of the several carrier waves traversing the entire system are caused to-be spaced away from the unmodulated or base fre'- quency component accompanying the side band. On account of the function of this additionalI auxiliary carrier wave, which may be of any relatively low frequency as, for example, from about 3,000 up to 10,000 cycles per second or higher, it is herein styled a spacing carrier or spacing carrier wave in order to distinguish it from other carrier waves used in the system.

It will later be shown how a spacing carrier wave having a frequency of 3.500 cycles per second may be employed to change the side bands of a speech modulated auxiliary carrier wave which, in the normal case, may be assumed to be of frequency of 50,000 cycles with side band frequencies ranging from 48,000 cycles to 49,800 cycles and 50,200 cycles to 52,000 cycles so thatvthe side frequencies will range from 46,700 cycles to 48,500 cycles and from 51,500 cycles to 53,300'cycles. As a result of this, it becomes practicalto receive a very small energy of auxiliary carrier frequency and select it from itsV side frequencies. After selection its energy is greatly increased by amplification whereupon it is recombined withthe unamplified or more slightly amplified side frequency Wave and impressed upon a detector.

A feature of the invention is that the auxiliary carrier frequency may be equal.-r

if desired, to the sum of the two principal carrier frequencies as an'alternative to the case Where it is equal to their difference. In special cases this may be advantageous.

Another feature consists in transmission of energy of the local transmitting generator into the local receivingl circuit without first passing it through the modulating alpparatus of the transmitter. In this way't e ratio of modulated and unmodulated energj.y of the outgoing principal carrier frequency which is impressed upon the local detecting apparatus may be regulated at theY will 'of the operator in anv manner necessary to secure the best `operation or as particular circumstances necessitate.

A further feature, the advantages of which' will be described hereinafter in detail, cnsists in selecting the intermediate carrier frequency and one or both of the accompanying side frequencies both before and after amplification.

A still further feature is the use, in a system employing -a partiallyy suppressed line.

Vcarrier and additionally employing any or wave, but alsoin' systems where a localsource of any suitable high frequency is used to beat with the incoming principal carrier wave to produce au lauxiliary carrier wave.

The novel system proposed admits of the use of certain desirable features'heretofore suggested in connection .with other systems,

among which may be mentioned :-(1) The p selection and transmission of one side band only at any point in' thesystem; (2) the connection of the transmitter and the receiver at a high frequency terminal station to a low frequency telephone line by means 0f a balanced or other suitable circuit whereby the radio signaling channel may be extended through one or more telephone exchanges to subscribers; suitable vselective circuits of other types than iterative band filters; and (4) low frequency select ing apparatus. y v

` Although the translating devices used for modulation, detection, and amplification are described as electron discharge tubes of the therniionic type it will be understood that the principles vof the invention may be employed in systems utilizing other equivalent forms of translating devices.

rIhe invention, will now be described in detail with reference to the accompanying drawings wherein F ig. 1 represents a complete radio terminal station comprising a transmitter and a receiver. connected respectively rto separate antennae; Fig. 2 represents alternative apparatus which may be substituted for the apparatus between the lines X-X and Y-Y 0f Fig. 1; Fig. 3 represents a simplified radio terminal station in which a spacing carrier is not used; and

Fig. 4 illustrates a convenient manner ofl connecting either of the stations illustrated Y in Figs. 1 or 3 to a low frequency telephone In Fig. 1 a signaling circuit 1 which represents any suitable source of audible fre- ,quency slgnaling currents is connected to the input circuit of a modulating system 2 of a type described, for exam le, in United States patent to Carson o. 1,343,306, granted June 1 5,V 1920. A source of spacing'carrier waves 3 is also connected to the modulatingr system. The output circuit of the device 2 is connected to the input circuit of a principal modulating system 4 of a type similar to the system 2.` Between the systems 2 and 4 is connected a suitable selective circuit 5 which is preferably a band-pass filter.

iso

supplied to the modulator 4, the output circuit of which is connected through a suitable selective circuit, preferably a` band-pass filter 7, to the input circuit of a power amplifier 8.. The vamplified wave ener produced by the amplifier 8 isoimpresse upon a transmitting antenna 9 of any convenient type.

' as source 3. The output circuit of the cir-- ,vergent channels.

The receiving system associated at a radio terminal with the transmitting system described consists of a receiving antenna 10 connected by suitable selective circuits to a detecting device 11, the output circuit of which is preferably connected through a selective circuit 12 and an amplifier 13 to two selective circuits 14 and 15, arranged in di- The waves passing through the circuit 15 are amplified in an amplifying system 16 which may cons ist of one tliermionic amplifier or a plurality of thermionic 'amplifiers or equivalent devices arranged in tandem. The divergent channels converge and are united by con-v necting output circuits ofthe devlces 14 and- 16 to the input circuit of a detecting circuit 17 in the output circuit of which is `a selective circuit 18 followed by another detecting circuit 19 to the input circuit of which is connected a source 3' of waves of spacing carrier frequency, that is, the same frequency cuit 19 is connected through a filter 20 which is preferably a low pass filter transmitting freely a range of frequencies from 0 to 2000 cycles and an amplifier 21 to a low frequency circuit 22 which contains -any suit-4 able indicating means, for example, 'a telephone head set or other receiver. .The selective circuits 12, 14, 15, 18 and 20 are preferably of the band-pass type described in United States patent to Campbell No. 1,227,113, granted May 22, 1917, but may be any selective circuits capable of performing, to a suiicient degree, the functions herein indicated. It is practicable to omit certain selective circuits, amplifiers, and other apparatus without rendering the system inoperative. However, the omission of these may render the system somewhat less selective or v less eilicientin some respects.

To explain the operation of the system, definite values of frequencies will beI stated. This, however, is-not intended to limit the invention to the particular values stated, since these may vary over a wide range. For telephony, the microphone circuit 1 will be considered as a source of speech frequency waves covering a range of from 200 to 2000 cycles of which the frequency is represented by a The Aspacing frequency source 3 has a frequency of 3500 cycles. The

vfilter 5 passes a range of frequencies of from suppress one of these side bands. It may or may not be designed to discriminate against the carrier frequency f1, but must in general permit the passage ofv some current of this frequency. The antenna -9 is preferably tuned to the mean of the frequency which it is to transmit but'may be tuned to discrimi nate against certain frequencies in favor of' others. From a distant transmitting station, which is supposed to be like the transmitting station illustrated in Fig. 1 with the exception that itscarrier frequency f2 (corresponding to f1) is 750,000 instead of 800,000 cycles, are received carrier waves. I will be seen that the incoming wave of freuency f., is accompanied by one or more side frequencies which result from modulation at the distant transmitter by means similar to those illustrated. In'the receiving antenna 10 a loop or trap circuit 23 is located which may be adjusted to discriminate to any desired extent against waves of frequency f1 and its accompanying side frequencies. The antenna circuit may consist of a tuned or untuned loop having the trap circuit 23 in series in the loop. The antenna 10 or an equivalent loop circuit may conveniently be arranged to have its maximum "receptivity in the direction of the distant station and its minimum receptivity in the direction of the antenna 9. A variable coni ico of frequencies f, and f2 'are combined by the detector 11 to produce waves of sum and difference frequencies, that is, fl-i-f, and )f1-f2, either of 'which may be utilized as an auxiliary carrier. Generally it is desirable toselect the lower of these frequencies for the auxiliary carrier, that is f1-f2 which in the present instance is a frequency of 50,000 cycles. This frequency will be designated as f3. The auxiliary carrier wave of frequency f2, will be modulated in accordance with whatever signaling waves are modulating the carrier wave of frequency f, at a given instant and likewise with whatever signaling waves are modulatliv? ing the carrier wave of frequency f2. Either party may at any time'break in and be heard by the other. The circuit 12 selects the auxiliary carrier and its accompanyingside frequencies and isv therefore adjusted to pass a band of from 46,700 to 53,300 cycles. It may, however, andpreferablydoes exclude one of the side frequencies. For exam-ple, if the lower side frequency is to be excluded, the band passed will extend from 50,000 to 53,300 cycles. The selected components are amplified by the amplifier 13and impressed upon two selective circuits -14 and 15. The circuits 14 and 15 are intended to separate side lfrequency energy from the energy of the auxiliary carrier freqency. Tliislallows'the energy of the auxiliary carrier frequency, that is. the unmodu- .lated component, to be amplified in, the "amplifying system'16 whereby its energy may be increased relative to that of the side Yfrequencies to any desired extent. The modulating system 4 is of the carrier suppression type and is intended to partially but not completely suppress the unmodulated component of the frequency f1, while at the dis- -tant transmitting station, a. similar lor equivalent modulating system will partially suppress the unmodulated component of the frequency f2. lNo definite extent to which the unmodulated component is suppressed need be assigned herein. -It may be assumed, for example, that` nine-tenths of the unmodulated component which would otherwise he transmitted', is suppressed. Instead of a band filter, the circuit 15 may consist of a series of sharply tuned circuits e'acli resonant to the frequency of 50,000 cycles and having thermionic amplifiers interposed between them. Such an arrangement is illustrated in the United States patent to AlexandersomrNo. 1,173,079, granted February 22, 1916. In. that case the elements of the selective circuit 15 and the amplifier 16 are divided up-and interposed between each other. The circuit 14 is adapted to pass the same range of frequencies-as the circuit -1'2 is adapted to pass; with' this distinction, however, that it may exclude the unmodulated component, that is, f,V or 50,000

' cycles, if desired'. On the other hand, the n unmodulated component may be transmitted through the circuit '14. The side frequency energy passing the circuit 14 and the ampli-I ied unmodulated component from the ampli-y fier 16 are impressed together and combined by the detector 17. In the output circuit of this detector, a component of energy results which has the same frequency rangev as that transmitted through the circuit 5, for example, 1500 to 3300 cycles, is selected by the circuit 18 and impressed upon the circuit 19 which has been variously designated in the art as a detecting, demodulating, or dis# torting circuit.

circuit 18 Without resupplying the spacing' carrier frequency. For this nnpose,' a source 3 is provided' wlnch impresses waves of the spaclng carrier frequency upon the modulating circuit 19. The sources 3 and 3 are of relatively low frequency and hence may be synchronized without much difficulty whereas if it should be attempted to supply .energy of the auxiliary carrier frequency, that is, f3, to the device 17 from a local source, more diiiculty would be experienced in vmaintaining synchronism within any specified range of allowable variation. The

speech frequency waves occurring in the circuit 1 or in the corresponding circuit at the distant station are reproduced in the output circuit of the system 19. These waves are preferably separated from interfering waves of higher frequencies by a filter 20, amplified by a device 21 and impressed upon the telephone circuit 22.

It will be observedothat thev circuit22 will receive not only the signal from the distant station but also the local outgoing signal.

The circuit 32 serves to supply a relatively small amount of energy of the outgoing car- 'rier frequency f1 to the detecting device 11.

The circuit 32 need' be only slightly coupled to the circuit ofthe source 6 and only slightly coupled to the circuit of the device 11. If undesirable lreaction occurs whereby energy is transferred from the input circuit of the device 11 to the circuit of the source 6, a thermionic amplifier 33 maybe included in the .circuit 32. This amplifier will repeat only in the direction away from the source 6. Since the couplingand adjustment of the circuit 32 and the adjustment of the device 33 are entirely under the control of the local operator,they may be adjusted from time to time in -any manner necessary or desirable to secure the best'results. The energy supplied through the circuit 32 is wholly unmodulated, which tends to the production of better quality and more uniform signals. The circuit 32 is not essential and may be omitted. When it is used, however, the spacial configuration and relationship of the antennae 9 and 10 and their tuning and adjustment may be such as to suppress from the circuit 10 the energy of the frequency f1 to a more` 45 i ed the signal can, nevertheless, be receivedmodulated carrier suppression modulating systemV 4. Furthermore, neither of these features is wholly dependent upon the use of a s acing carrier frequency or upon a modu. lating system 2 of the carrier suppression type.

Means for superimposing a telegraph or call channel upon the telephone channel is illustrated. A source t ofcurrent having a suitable frequency is connected to the input circuit of the modulator 4 by a circuit including a key. When the key is closed the' carrier frequency f1v is modulated by gares from-the source t. If the source t has any frequency without the range of the band 1500 to 3300 cycles, it will interfere little with the speech-transmission. When the `call channel is used in this manner the 'pass various filters through vwhich the resultant modulated wave passed will. be modified4 to pass the additional sidel frequency thus produced. Forl example, if t has a frequency of 3400 cycles per second, the filters 7, 12,

14 and 18 must be designed or adjusted to a frequency removedv 3400 cycles from the carrier. A circuit'34 tuned to 3400 cycles and connected to an alternating current relay 35'or equivalent receiving instrument serves for reception of the telegraph or call signal.' The resent system offers considerable possibilities in the way cf making the unauthorized reception of signals difiicult.4 Partial suppression of the unmodulated component and transmission in each direction on different carrier waves are two fea' tures which tend to prevent reception by simple systems. Let it be assumed' that the present system radiates the upper side band resulting from the modulation of a' carrier "vof800,000cycles with a side band of1500 to 3300A cycles., Then this signal can be picked up by means of a local source of 803,4 500 cycles. If both. side bands of the 800,000 cycle wave, modulatedv as stated, are -radia'tby' means of a properly vselective antenna and a local source. Ifhowever,"thetra ns' mitted. carrierv waveA variesv over a range of f several hundred cycles, such unauthorizedv reception becomes more difficult. For secrecy this variation may be 'purposely-intro p duced. Let it be assumed that the local and distant principal carriers are each varying from 250 cycles above'the mean value to 250 cycles below the mean value. e If the variations of -the'principal carriers are not synchronous, a total variation of the intermediate carrier of from 500 cycles above to 500 cycles below the Q mean lvalue will occur.

Y For transmission of a single sideband, that is, upper .side band, the following. filter [values will be needed: filter 7, 799,750 to y803,550 cycles; filter 12, 49,500 `to 53,800 cycles; filter 14, .51,000 ,to 53,800 cycles; andA filters 5, 15, 18 and 20 would have the values lndicated on the drawing. For .transmission of both side bands, ,the values would' be:

`call frequency of 3400 cycles can still be uscd. In case this is used each filter throu h which the wave modulated with 3400 cyc es passes must have such a value as to pass a component differing from the carrier by 3400 cycles. Ifone carrier is maintained constant, the other may be -given a total variation of from 500 cycles above to 500 cycles below the mean carrier frequency. In other words, the sum of the total variations of both carriers from their mean values determines the filter values necessary in the receiver circuit. This variation may bedivided between the two carriers in any desired proportion. If the variation is an accidental one instead of intentionally. produced, the results will be the same.

In Fig. 2, a selective circuit 12 is connected between the amplifier 13 and the selective circuits 14 and 15. The purpose of this arrangement is to give additional amlification and selectivity. The source 27 -lndicates the introduction of anadditional iliary carrier frequency is to be stepped up to a higher value, for example, from 50,000 cycles to 150,000 cycles. In this case, the source 27 may have afrequencyjfof 100,000 cycles designated as f4. This need-not be accurately maintained Ias a slight frequency variation will ordinarly produce no detrimental effect. The modulating device 28 serves to combine the energy transmitted throu'glr the filter 12 with energy from the source 27 .thereby producing a supplemental auxiliary carrie-i' frequency `of 150,000 4as wellv as a difference carrier frequency of 50,000- which will be ignored as 1t is supi The. 150,000 cycle component is, of course, accompanied 'b 4'one or both lside bands as is determine by the transmission of one or bands from the local and ldistant station and the corresponding adjustment of the circuits12 and 12. Naturallywhen both side bandsaare utilized at one point in the system it is desirable that-both be transmitted throu h cuits,I and when only one is transmitted, then the filter circuits will be adjusted so ressed by the circuits 14- and 15.

all the. various filter cirthat only the corresponding` side band will v be transmitted and other frequencies supbands.

pressed. .The advantages of the transmission of only one side band are well known. The present invention is applicable to sthe transmission of either one or both side The circuits 14'and' 15 in Fig. 2 are analogous to the circuits 14 and :15 in Fig. ,1 and performfid'entical functions.

' Their difference consists only in such design or adjustment as is necessary to render them applicable to a carrier fre uency of 150,000A

The ap acycles instead of 50,000 cy les. f

ratus between the lines X-X and Y- Figs. land 2 are interchangeable and may` be substituted each` for the other.

In Fig. 3 is illustrated a simplified system wherein a spacing carrier source is not utilized. In this system the carrier fre-- quency'f,l is directly modulated by a speech or other signaling wave and the resultant energy impressed directly or through amplifiers (not shown) upon atransmitting antenna 9. The receiving antenna 10 is .similar to that vof Fig. 1 and the detecting selected, the amplifying system 13 and the associatedfilters 12 and 14 are designed to select and amplify the auxiliary carrier frequency fl-I-fz-l-s. The double sign before .3 indicates that either `oneor both side frequencies may be selected. On the other hand, the system 12, 13, 14 may be designed and adjusted to select and amplify the difference frequency, that is, fl-ff-s. The, various ranges of frequencies which may be passed by the filters 12 and 14 are Iindicatedon the drawing. .The Vdetecting circuit'v 19 detects the. waves transmitted through, the filter 14 thereby reproducing theoriginal'speech waves which are then limpressed upon al telephone receiver circuit. 22. 'In' the arrangement of-Fig. 3, as so. fardescribed,.the modulator 4 does not Vsuppress the unmodulated component or case it may be supplied through the circuit4v v' 32-as described in connection with Fig. 1.

bythe device 19 is still possible.

'suppresses it'` to such a small extent that reproduction of theforiginal signaling wave If desired, however, the modulator 4 at. the local station' may suppress the unmodulated component as completely as possible, in which In case the corresponding modulator 4 at the distant cooperating 'station partially suppresses the unmodulated component, then it becomes necessary to substitute for the system 12,13, 14 of Fig. 3, an arrangement like thatvof Fig. `1, consisting of the elements 12, 13, 14, 15 and 16, since otherwise the energy of the unmodulated auxiliary carrier frequency in the input circuit of the device 19 would be too small to permit good signal reproduction.

Fig. 4 illustrates the manner of connecting a low frequency telephone line 29 tothe inputTcircuit of the local radio transmitter and to the output circuit of-a local radio receiver so that a two-way conversation may be carried on via radio by a subscriber at the remote end (not shown) of theline 29. The transformer 30 represents the trans` former 30 of Fig. 1, and the amplifier 21Y represents the amplifier 21 of Fig. 1. The network N balances the line 29. at speech frequenciesand the circuits leading to and from the amplifier 21 andthe transformer 30 arev connected in conjugate relation with respect to each other to the triple windingr transformer 31. The systems illustrated in Figs. 1, 2 and 3 or any equivalent system may be connected to a telephone line inv a similar manner. *Y

The object of providing duplicate filters 12v and 14 needs brief mention. Assume that the filter 14 is omitted. Incoming impulses would tend to act upon the filter 12 and to set up oscillations having all the-possible natural frequencies of the filter. If any of these natural frequencies or possible "combination frequencies which might be produced by distortion in amplifier 13 or by detector 14 are within the range passed by the filter 18 they will appear in the line 22 as noise currents. -When filter '14 is included, an additional discrimination yoccurs ir that disturbing foreign frequencies from impulsive excitation of filter 12 or distortion in amplifier 13 must not only pass through filter 14 but their combination frequencies must likewise pass vfilter 18. Assuming filter`12 to be omitted and filter 14 presresulting ent, then impulsive excitation of filter 14' will cause somewhat similar disturbances and in addition foreign frequencies which cannot pass filter 14 may combine in amplifier 1'3-owing to the inherent distortionto produce waves which will pass filter 14.-.

filter 14 and the undesired waves .produced by filter 12 and `amplifier 13 are largely supf 'pressed by filter 14. Another-advantage of lter 12 is an increase of efficiency in the amplifier `13 in consequence of permitting it to handle lessenergy of undeslred and unused frequency components. Likewise when `filter12 'is provided any incoming waves which are such as can produce combination frequencies capable of passing filter 14 will 1 not be transmitted -to amplifier 13 to cause.

j Thus, animportant feature of the present invention consuch combination frequencies.

sists 1n selecting the intermediate frequency 130 band and the unmodulated component of the auxifiary carrier frequency both beforel and after amplification. The filters used should have sharp and definite cut-off limits if high efficiency and good selectivity are desired. Double selection of the intermediate frequency is not dependent on the use of, a spacing carrier 4or of ulimodulated carrier suppression means for its utility.

The use of a spacing carrier permits tlie auxiliary carrier vfrequency to be spaced away from its accompanying side bands by any necessary, amount,I This enables the auxiliary carrier to be selected by means of apparatus of moderate cost and reasonable refinement. Unless the auxiliary carrier can be selected from its accompanying side frequencies, it is difficult to, introduce any great amplification by the amplifier .16 because if some of the side frequency components are amplified in the device 16 and others are not amplified, or are only slightly amplified, an

undesirable distortion is introduced as a re.-

sult of increasing the -amplitude of certain y speech components disproportionately.

If the 'transmitting modulator 4- and the amplifier 8 are not operated during al time when the receiving system is receiving waves from a' distant station, but the source 6 iS operating and is supplying energy through the circuit 32 to thedevice 11,'it will beseen that the auxiliary carrier frequency will 'be produced, selected, amplified and detected in the same manner as if the transmitting station is operating. The source 6 then acts as a local source used solely for aiding'reception and not for' transmission.

For line wire transmission, the antennae 9 Aand 10 are replaced by' any suitable-wire circuit extending to the distant station.

Before pointing out the` novel features of the invention 'inthe claims, the scope of certain expressions used therein will be indicated. The expression side frequencies o r side band includes a single sidefrequency as well las a band of side frequencies. Unlmodulated component is an expression i adopted from British Patent No. 102,503, of

1916 and is used in the same sense as in that patent.' However, itis not intended to limit the present invention to the type of modulator described in that patent. The suppresv sion ofthe carrier may occur in the modulator itself or result from the action of selec- -tive circuits or .partially from both.

The novel features believed to be inherent lin the invention are defined in the appended claims. 'i

What is claimed is: -V

1. The method of securing the advantages of partial suppression of the unmodulated 'component in a wave transmission, sys-- rein employing a detector, which comprises transmitting only feeble energy of the unmodulated component, separating the unmodulated component from its accompanying side frequencies at a point in the .sys-- tem where the total energy is small relative to the transmitted energy, increasing the 1energy of the selected unmodulated component, and combining at .the detector the increased energy of the unmodulated component with its accompanying side frequencies.

2; The method of signaling in a system wherein 1s transmitted' a carrier Wave modulated in accordance with a signal and having an unmodulated component toofsmall,v

to reproduce a sufficiently perfect signal, which comprises receiving, selecting out as such, and increasing the energy of thel unmodulated component at a receiving station,

and recom-bining, the unmodulated compov 'nent of increased energy with the modulated carrier wave energy to produce a modulated carrier `wave having -a relatively greaterun- Vmodulated v.componefntthan the received wave.

3. The method which comprisesv largely suppressing the unmodulated component of a modulated carrier wave at a transmitter,

receiving the energy of the unmodulated component and one or more accompanying side frequencies; and yproducing energy de- Y 'spect to the received side frequencyenergy.

4. The method which comprises transmittingva wave of relatively small amplitudev and a side frequency of relatively large amplitude of a modulated Wave, receiving the energy of both, deriving energy yfrom the side frequency, deriving energy from the first mentioned .wave having a relatively niol muchgreaterratio to the received unmodulated component, and combining said derived energies. r y

5. The method which comprises largely suppressing the unmodulated component'o a modulated carrier wave at a transmitter,

receiving the energy of the unmodulated component and one or more accompanying side frequencies, converting the frequencies of all thereceived waves by an equal amount to -produce another unmodulated componentl and, one or more accompanying side frequencies, and producing from thelconverted ulmodulated` component, energy having .its relative amplitude proportionately greater is with respect to the received side frequency with respect to the side frequency energy. than the' received unmodulated component energyi1 d y l f ff v 6. e method which comprises transmitting an unmodulated component of'relatively small amplitudeand a side frequency of relatively large amplitude` receiving and converting the energy of both to produce an auxiliary unmodulated lcomponent of small amplitude and an auxiliary side frequency, amplifying'in relation to the modulated component the energy of the unmodulated component, and combining the resultant energies.

7. 'The method which comprises, successively detecting an unmodulated component and an accompanying side frequency and converting the frequencies thereof, and at one stage amplifying one of the energies representing the unmodulated component and the modulated component disproportionately with respect to the other.

8. In a signaling system, means for transmitting an unmodulated component of rela' tively small amplitude and a side frequency of relatively large amplitude, means for .receiving'the energy of both, means for converting the frequencies of both by a fixed amount, means for increasing the energy of the unmodulatedlco'mponent to a relatively high value with respect to theL modulated component, and means for combining the amplified unmodulated'component and the Y modulated component'A to reproduce a signal-- ing component.-

9. In a signaling system, a transmitter includ-ing means for partially suppressing the unmodulated component of the transmitted wave,'means for receiving" the energy of the feeble unmodulated component and one or more accompanying side frequencies,means for deriving an unmodulated `component of increased amplitude from the unmodulated vreceived component, and means for combining the side frequency energy with the unmodulated component of relatively increased amplitude. v

10. A receiving system comprising a detector for combining incoming carrier Waves with ,locally generated waves to roduce an auxiliary carrier wave, a selective circuit for selecting the unmodulated component of the auxiliary carrier 'wave from the modulated components thereof, means for amplifying the selected unmodulated component,

and means for recombining the amplified unmodulated component' with the modulated components. l l.

11. A signal transmission system comprising two cooperating stations, each transmitting to the other and each supplying high frequency energy of its own carrier frequency to its local receiver, means at each of said sta- `tions to transmit a modulated wave having a relatively small unmodulated component, .and means at each receiving station to' increase the energy of the unmodulated component to an extent disproportionate to any lll'ee Q the' modulated component,

mitting to the other and each supplying high frequency energy of its own carrler frequency to its local receiver, frequency changing means at each station for combining the energies received from the local and distant stations and converting the combined energies *to an auxiliary carrier frequency energy, means at one of said stations for partially suppressing the unmodulated component of the transmitted carrier frequency energy, and means at one of said stations to increase the energy of the unmodulated component of the auxiliary carrier frequency energy disproportionately with respect to any increase in energy of the modulated component. y I

13. In a signaling system, a transmitter comprising a source of high frequency waves, a source of signaling waves, and a spacing carrier source for spacing the side frequencies of the modulated carrier wave away from the unmodulated component in combination with a receiver having means for separating the side frequencies from the unmodulated carrier wave.

14. In combination with a receiving system comprising means for producing from received carried waves an auxiliary carrier wave, means for 'selecting the unmodulated component of the lauxiliary carrier wave from the modulatedcomponent, means lfor amplifying the selected unmodulated com onent, means for recombining the -ampli ed unmodulated component and the modulated component, and means for combining energy ofthe spacing carrier frequency with the waves produced bythe lastmentioned combining means.

15. A system comprising 'two cooperating stations, each provided with means for transmitting its own carrier wave to the other and. each provided with means for re- 16. In a system in accordance with c1aim l 15, means for` decreasing by a substantial amount the amplitude of the unmodulated component of the carrier wave transmitted fro/m the other station.

17. In a signaling system, a source of signaling waves, a spacing carrier source, means ,for modulating the spacing carrier source with waves from the source' of signaling waves, a principal carrier source, means for modulating waves from the principal carrier source with waves .resulting from the first mentioned modulation, the system being devsigned and arranged to partially suppress c arrier component. of the transmitted principal carrier, and'means at a cooperating receiving station for combiningenergy of the spacing 'carrier frequency with the receivedwaves.

1 8. A signalin system comprising a receiving channel or'incoming'high-frequen- A f ingtheunmodulated component of increased,

4 5 ergy toproduce ainodulated cy waves, means for supplyingl locally generated waves to the receiving channel, means for combining .energy of the received and locally generated frequencies to .produce an auxiliary carrier `frequency of lfrequency higher than the 'frequency of the combined v Wayes and accompanying side frequencies,

and means for ampli ing the unmodulated component of the auxiliary carrier frequen vc without a corresponding amplification of' t e side frequency energy, and means for' combinin the amplified umnodulated component with .the sidev frequency energy.

v19. A signaling system' comprising twoY cooperating stations, each provided ,with

means for, transmitting a modulated carrier wave, means at one o f said stations forl combining a portionof` the energy of the outgoing carrier wave source with energy inc omin v from the other station to roduce anauxi ary carrier frequency equa tothesum of thetw 'carrier frequencies, means for selecting the sum frequency, and means for deriving. the signal to bereceived from v energy with the modulated carrier wave encarrier wave having a relatively greater unmodulated `:component than the received wave.

vlili .cooperating stations, each 21. A signaling system comprising two rovided with means for-transmitting to t e other a vsignal modulated high uency wave, one of said stations being provided with means for combining energy of the. incoming and outing carrier frequencies to produce an auxary carrier frequenc and one of said stations which may be t e same or the other station having means for supplying to thisl Y receiving circuit unvaried carrier frequenc erated energy to energy at all times whether the local signa in apparatus is operating or not.

g2. a wave receiving system, means for combining received energy with locally genproduce an auxiliary carrier wave, amplifying means for the auxiliary carrier7 wave selecting means both preceding and. folle lto produce an auxi lthe unmodulated component o vcarrier |wave.

26. In a receivin system, means for selecting a carrier an an accompanying side means, said selecting 'means each comprising a band pass lilter.

23. The method of signal reception which comprises converting a received carrier wave iar carrier wave, and selecting the carrier an one side band of the auxiliary carrier Wave to v'the exclusion of the other side band bothA before and after amplification.

24. A signaling system comprising two 'cooperating stations. each having means to transmit a signalmodulated wave to the other, one of said stations having means for lreceiving energy from a distant transmitter` and combini it with' ener of the local outgoing carrier .wave to pro uce an auxiliar carrier wave and means for selecting an amplifying th auxiliary carrier wave comprising aselective circuit, an amplifieran'da second selective circuit in tandem arrangement lin the order'named.

. 25. In signal reception, the method which comprisesV producin an auxiliary carrier wave from a receive carrier wave, selecting the unmodulated component of the auxili carrier wave and a side band before amphfying, am lifying the selected com onents, then selecting the side band sefparatelly from the auxiliary susl band before amplifying, means for ampli'- Hfying .the vselected components, means for separating the unmodulated 'component of the carrier from the side band, and means for l amplifying the selected unmodulated component separately from the side band.A

27. The method of introducing an elementl ofi secrecy into carrier wave, signal transmission which comprises transmitting a side band based upon a given carrier wavewhich is variable in frequency transmitting with the side band energy of the variable carrier wave which is of too smallintensity to reproduce a signal of satisfactory intelligibility byA direct combination with the side and, receiving said side -band and carrier wave, producing from the received energy a wave correspondin tothe carrier wave and of energy great y increased with respect. .to the energy of the side band, and

combining the side band wave and the -produced wave. Y

28. A signalling system comprising a high frequency section connected to a te ephone line by means of ahigh frequency terminal,

said high frequency termina ed to said te e hone line by means of conjugately relate input and output. voice frequency circuits; said input circuit vbeing connected to a carrier suppression modulator, a spacing frequenc source connected to said modulator, a ban filter in the output of s 'd modulator adapted to suppress `waves being connectv iso having the frequency of said spacing frequency source, a second carrier suppression .modulator supplied by said band filter, a`

principal carrier frequency source supplying said last mentioned modulator, a band filter for selecting a component from the output of Said last mentioned modulator, a power amplifier connected with said last mentioned band filter, a transmitting conductor supplied by said power amplifier; a high frequency receiving circuit terminating in said output circuit, said receiving circuit comprising in tandem relation, Vselecting means, (2) a detecting device, (3) a band filter, (4) an amplifying device, (5) a twobranched parallel combination consisting of a band Elter in each branch and an amplifier in one branch, (6) a detecting device connected to Aboth of said branches, (7) a band filter connected in the output of said detecting device, (8) a detecting circuit connected to the output of said band filter, (9) a source of spacing frequency waves connected .to said detecting circuit, and (10) a band filter for passing speech frequencies connected to said lastmentioned detecting circuit, said band lter leading into saidoutput circuit.-

29. A station comprising means for transmitting a carrier wave, means fox` combining received energy with energy of the local outgoing carrier wave to produce an'auxiliary wave, amplifying means for the auxiliary carrier wave, and selecting means both preceding and following said amplifying means said selecting means each comprising a band pass filter.

30. In a. signalingsystem, means for combining incoming and outgoing carrier waves to produce an auxiliary carrier wave in combination with means arranged in the order named for selecting, amplifying, and .again selecting the auxiliary. carrier wave energy before detecting it.

31. A two-way signaling system comprisingfmeans for receiving and combining the energy of an vincoming carrier wave with energy of the outgoing carrier wave frequency, means for selecting from the resultant waves a modulated component which is of a frequency equal to the difference of the incoming and outgoing carrier frequencies, means for detecting the selected component to produce an audible frequency wave, and a non-Signaling channel for supplying energy of the local outgoing carrier frequency to said means for combining.

32. A two-way signaling system comprising means for receiving and combining the energy of an incoming carrier wave with energy of the outgoing carrier wave frequency, means for lecting -from the resultant waves a modulated component which is of a frequency equal to the difference of the incomingvr and outgoing carrier frequencies, means for detecting the selected component to produce an vaudible frequency wave, and a non-signaling channel for supplying only the unmodulated carrier component of the outgoing carrier wave to said means for combining. f

In witness whereof, I hereunto subscribe my name this 31st day of January, A.,D. 1922.

. JOHN F. FARRINGTON. 

